First, here is my console log for my MP server https://gist.github.com/Gunner76th/cc5af071cd7599c612ca

The spam starts with the following line, and continues until I shut down the server:
[08:35:31] [Server thread/INFO] [STDERR/]: [java.lang.Throwable$WrappedPrintStream:println:748]: java.lang.ClassCastException: gravestone.tileentity.TileEntityGSSkullCandle cannot be cast to gravestone.tileentity.TileEntityGSGraveStone

When the spam started happening, was when I began looking at a gravestone that use to be a Skull candle, but for some reason was changed to a gravestone. If you attempt to break the gravestone, the client drops from the server, and receives the message "Internal Server Error" The only method that I have found to remove the bad blocks, is to download the world folder, load the world in McEdit, and delete the blocks from the world.

At first I thought this was an issue with Witching Gadgets, but when the "broken" graves still existed after that mod was removed, and after having examined the console log while interacting with said blocks in game, it would appear that they are in fact caused by some sort of NBT or block ID corruption from the Gravestone mod. The server has not suffered any unexpected shutdowns or power losses that would generally cause data corruption. Prior to the "broken" blocks appearing, I had done a world trim to reset all unoccupied chunks in our world so that they could regenerate with world generation from one of the mods. Right afterwards, various graves and Gravestone mod items within the game changed to other objects (candles at one location became ender chests, a villager statue became a mirror from Thaumcraft, skull candles in my wife's base became stone graves), and trying to remove these objects results in the client disconnecting and an "Internal Server Error" message to be received.